Chapter 25. The H (Headers) Configuration Command

All mail messages are composed of two distinct parts: the header
(containing information such as who the message is from) and the body
(the actual text of the message). The two parts are separated from
each other by a single blank line (although there are exceptions,
which we will cover). The header part used by
sendmail was originally defined by RFC822 (with
clarifications contained in RFC1123), and most recently defined in
RFC2822. These three documents detail the required syntax and
contents of most header lines in mail messages. Many other RFCs
define other headers, but in this chapter we will discuss header
lines as they relate specifically to sendmail,
referencing other RFCs as necessary.

When sendmail receives a mail message, it
gathers all the header lines from that message and saves them
internally. Then, during queueing and delivery, it re-creates them
and augments them with any new ones that might be required either by
the configuration file or by
sendmail's internal logic.